Hot melt one component thremosettable epoxy resin compositions are difficult and hazardous to manufacture and pack. These difficulties often give rise to variability in the product both from batch to batch, container to container and even within containers.
These problems arise because the compositions contain viscous or solid resins as well as hardeners and are mixed in the hot viscous molten state. This can result in the reaction fully or partially occurring during mixing, discharging, or even in the end container.
This invention relates to methods utilizing a specific type of composition for the safe and consistent manufacture and where required the simple filling of end use containers of one component heat curable, solvent free, hot melt epoxy resin compositions which have a melting point less than 55xc2x0 C.
Because of safety, cost and possible environmental pollution it is becoming increasingly important to avoid the use of volatile solvents in industrial processes. Because of the need to maintain a safe, clean and healthy workplace, as well as manufacturing convenience, contact with all liquid chemicals should be avoided wherever possible.
These needs favour the use of epoxy resin formulations in solvent free, flexible or solid form and account in part for the increasing popularity of preimpregnated fibres, or prepregs, for the manufacture of reinforced composites, adhesives in hot melt and flexible tape form for bonding and the use of powders for coatings.
It is very important that the physical nature of these epoxy formulations is very consistent during storage because this affects the ability to apply them reliably and produce consistent quality items with them.
Mixing of the epoxy formulation does not present manufacturing problems is made in solution, as the ingredients can be mixed at low temperatures to avoid dangerous reaction between the resins and hardeners present, due to the low mix viscosities achieved by the use of solvents, but the manufacture and packaging of solvent free individual batches of reactive hot melt epoxy resin and hardener is much more difficult and dangerous, especially when the final composition is required to cure at temperatures say up to 180xc2x0 C., due to the risk of the curing reaction being initiated prematurely.
Alternatively, mixing of the ingredients can be carried out continuously, which usually avoids the danger of major heat or reaction being given out in the blending equipment, even if carried out at elevated temperatures because only small quantities are present in the mixer at any one time. But the danger remains of reaction in the containers the mixture is poured into.
The result of reaction may be very serious where it proceeds out of control, leading to large quantities of decomposition gases, burnt products, damaged equipment and harzards to personnel, the workplace and the environment in general. Where the reaction only partly proceeds it leads to an increase of average molecular weight, either in the whole batch, or variably within a batch, or a container, for example as the result of different heating times coming from the variation in run-out times from the mixing vessel or cooling within a container. Any such partial reaction is very bad as it results in changed application and uncured physical properties either of the whole batch or container or, more likely, variability within it.
Because of these difficulties the volumes of many hot melt epoxy compositions that can safely be mixed at one time or be put hot into a compact bulk in a container rarely exceed 100 liters. Where the final composition is required to cure at 130xc2x0 C. or below even 25 liter mixes are potentially dangerous.
Whether mixed continuously or batchwise the filling of small containers with hot viscous reactive liquids present major problems, particularly so when the viscosities at the permitted filling temperatures are high.
So a new method was needed to manufacture these difficult and hazardous materials both safely and in reproducible quality, including their presentation in practical end use containers.
We have now discovered a surprisingly simple way to make these one component epoxy hot melts consistently and safely and supply them in a wide variety of containers and shapes suitable for later hot melt or other processing applications. This process uses very mild conditions and consists of making an epoxy formulation which is liquid at 80xc2x0 C. or below, more usually at normal shop floor temperatures (15xc2x0 C.-30xc2x0 C.) and adding to it a chemical solidifying system which reacts very slowly at these temperatures with the epoxy materials present.
The solidifying system must be picked to give very little reaction during the time it is being mixed with the epoxy resin and hardeners, by whatever method this is done, so that there is very little viscosity rise or temperature rise during the blending operation and hence making the filling of large or small, simple or complicated containers a relatively easy task. Alternatively mixing may take place in the final container if required.
The solidifying reaction must be a simple amine addition reaction with the epoxy groups and must stop when the addition reaction stops. No tertiary amines may be present in the initial mixture or generated during the reaction which could significantly react under the conditions chosen for the solidification reaction. Such reactions severely compromise, safety during bulk mixing, the state of solidification once mixed and the melting point stability and shelf life of the resultant product. The solidifying system must be picked to satisfy these criteria.
The levels of solidifying system used have a major effect on the physical nature of the fully reacted, uncured product. With a liquid resin increasing amounts of solidifying systems take the product through the stages of high viscosity, high tack, low tack, zero tack, flexibility and brittleness respectively.
For convenience, most of the solidification reaction should take place in the final containers used to receive the mixture and this should take place slowly enough to ensure that, in the selected size and shape of the container, there is no temperature rise high enough to cause any significant reaction between the epoxy resins and their main curing systems, but fast enough to ensure useability in a sensible time.
The solidifying agent may be introduced into the mixture at any stage in the process provided the reaction basically proceeds as above.
As the necessary quantity of the solidification system reacts the viscosity and melting point of the formulation increases until the reaction approaches completion when the physical nature of the final formulation is close to the desired end use requirements.
The present invention provides for the manufacture of one component hot meltable thermosettable epoxy resin formulations in bulk form in batch or continuous mixes and their presentation in containers intended for further processing by melt, or other, application methods and includes their composition in hot meltable, molten, powdered, solid, semi solid, tacky, bulk and final cured forms. For example the method of this invention could be performed using the following classes of ingredients:
(A) epoxy resins or compound containing more than one epoxy group compounds
(B) a solidifying amine system which will react with (A) to give a product with a Kofler Heat Bank melting point of less than 55xc2x0 C., but which is not present in sufficient quantities to allow or cause chemical gelation under the reaction conditions chosen for (A) and (B) and which essentially stops solidifying once its active epoxy additive hydrogen groups are consumed by the epoxy groups, optionally
(C) a hardener system for (A) and the reaction product of (A) and (B) which is different from (B) and which remains substantially unreacted under the conditions of reaction chosen for (A) and (B), optionally
(D) other additives that may be required to modify the physical properties of the cured or uncured composition, and optionally (E) an expanding agent.
The method is carried out by blending (A), (B), and optionally (C), (D) and (E) together by any convenient batch or continuous operation but in such a way that at least (A) and (B) become homogeneous. The reaction between (A) and (B) may be carried out at any suitable temperature and condition provided that neither it, nor the exothermic heat generated from it causes (C) or (E) to substantially react whilst it is taking place.
The epoxy resins or compound containing more than one epoxy group compounds, (A) employed in this invention may be glycidyl ethers, glycidyl amines, glycidyl esters or cycloaliphatic glycidyl compounds, or combinations of these including halogenated versions where required. Preferred epoxy resins and blends are those which are suitable liquids for ready mixing with the other ingredients at suitable temperatures which will usually be below 100xc2x0 C. Epoxy resins or compound containing more than one epoxy group compounds or blends of them which are liquid at room temperatures are the most convenient.
The preferred solidifying systems (B) used to convert the liquid resins are principally compounds or mixtures of compounds whose most reactive groups relative to the epoxy materials employed are primary or secondary amines. Epoxy reactive tertiary amines under the conditions of reaction chosen for (A) and (B) are not acceptable for this invention.
Of particular usefulness in this process are aromatic and cycloaliphatic primary and secondary amines and blends of these. The major advantage of these amines, particularly the aromatic amines, is the low rate of reactivity coupled with the extremely long life at normal ambient temperatures of their reaction products with the resins. With the majority of compounds from these classes of amines the life of the reaction product with the resins greatly exceeds that of the life of the resins with their primary hardeners (C). Some alicyclic, heterocyclic and aliphatic amines are also effective as advancing agents and those which comply with virtual cessation of reaction once their amino hydrogen atoms have been consumed by the epoxy resins are considered as part of this invention. In all cases it is essential that the tertiary amines generated during the solidification reaction have very low reactivity with epoxy groups under the conditions of reaction chosen for (A) and (B) and afterwards during storage. The solidifying amines are usually and mostly difunctional and/or polyfunctional with respect to the epoxy compounds, (A), although monofunctional amines can be used to some extent if of value to a particular composition.
Difunctional amines may be used at any desired ratio with difunctional epoxy resins but greater than difunctional amines only to levels where gelation does not occur. The solidifying systems may contain a variety of other groups but these should only be of very low or no reactivity towards the epoxy groups involved under the reaction of (A) with (B).
The hardener systems (C) for component (A) and the reaction products between (A) and (B) can be selected from the wide variety of those well known in the field of epoxy chemistry other than acid anhydrides which react preferentially with the advancing agents (B). Typical but not exclusive examples of useful hardeners are aromatic amines such as diaminodiphenyl sulphones, boron trifluoride amine complexes, latent imidazoles, polycarboxylic acids, polyhydrazides, dicyandiamide, latent epoxy amine adducts and substituted ureas. As explained a main requirement of the hardener is that it should not substantially react whilst (A) and (B) are being reacted to form the epoxy composition which has a melting point less than 1 20xc2x0 C. There may be one or several hardeners used together, some of which may accelerate the curing rates of the others provided they comply with the requirement immediately above.
Other additives (D) which can be used to modify the physical properties of the cured or uncured compositions include but are not limited to thixotropes, toughening agents, wetting agents, surfactants, fibrous materials, dyes, pigments, fillers, flame retardants, smoke suppressants, coupling agents, hollow microspheres, flow assisting materials, fusible glasses, expanding agents and stabilisers.
Suitable expanding agents are those which generate gases by chemical decomposition or by boiling of liquids or expansion of gases contained within exandable shells.
Examples of suitable expanding agents include Azodicarbonamide, Azodiisobutyronitrile, Benzene sulphonhydrazide, Dinitroso pentamethylene tetramine, Oxybis benzene sulphonhydrazide, p toluene sulphonyl hydrazide and Expandable plastic such as those sold under the Trade Name Expancel. These are largely spherical shells of varying composition such as polyvinylidene chloride and or polyacrylonitrile plus other copolymerised additives, and the inside contains isopentane xc2x1air.
It will be clear to those familiar with epoxy resins that the actual mixing and storage temperatures, the geometry and volume of the mixing vessel and the containers the mixing and filling times required as well as the actual resins and the quantity of them used will all influence the selection of the solidifying agents. It would not be good for instance to choose a solidifying agent which reacts to generate substantial heat during the mixing operation or in the selected container shape and size. Thus the less reactive amines are the most suitable solidifying agents for practical batches and containers whereas most aliphatic amines are unsuitable alone, because they are highly reactive.
Most useful are those solidifying systems which react gradually to substantial completion at room temperatures over a period of around 2-14 days. These permit the safe manufacture of batches in excess of 100 liters in a realistic mixing time with little temperature rise in the mixing vessel or during discharge and smooth reaction to the required physical state in most practical containers, however mixed, over a practical timescale. Under these conditions the heat of reaction generated by the soldification process is evenly dissipated by conduction and radiation and results in no more than acceptable temperature rises at any stage in the process.
The primary controlling factor being that the mixture reaction temperature rise whether in the mixing vessel or the containers shall be below that required to cause significant reaction between (A) and (C).
Should it be desirable to speed the solidification in the final container this can be achieved by heating, provided the temperature used does not cause significant reaction of (C) with (A) or the reaction product of (A) with (B) either by direct heat or that evolved by completing the reaction between (A) and (B), or by the addition of accelerators such as carboxylic acids, which do not adversely affect the softening point stability.
The solidifying systems must be present in such quantities that when their amino hydrogen atoms are all substantially reacted with the epoxy materials (A) under the conditions set for reacting (A) and (B) the product is not chemically gelled and has a melting point which is essentially stable for greater than 6 months at 22xc2x0 C.
The selection and quantity of the solidifying agent will also influence a variety of properties such as melt viscosity, stength, toughness and heat resistance and by careful choice advantages may be designed into the uncured or cured products resulting from the use of this process.